Radio transmitters are utilized to transmit information over the air. This is typically accomplished by modulating a carrier signal with an information signal. For amplitude modulated signals, the amplitude of the carrier signal varies with the amplitude of the information signal. Alternatively, for frequency modulated signals, the frequency of the carrier signal varies with the frequency of the information signal. Typically, in frequency modulation schemes, a phase locked loop (PLL) is employed with a fixed reference frequency signal, perhaps provided by a crystal oscillator, to synthesize the carrier frequency. The voltage controlled oscillator (VCO) of the PLL is directly modulated by the information signal to result in the modulated carrier, which is then provided to an antenna for transmission over the air.
In systems requiring DC response, that is when the information signal contains very low frequency components, the reference frequency signal is also modulated with an additional signal then provided to the PLL. This additional signal varies with the information to be transmitted and is employed to compensate for the non-DC response of the PLL. In other words, the use of the additional signal prevents the PLL from tracking out the information signal when the information signal is low in frequency.
A common problem resulting from the use of an additional signal to prevent the PLL from correcting out the information signal, however, is that mismatch can occur between the additional signal and the information signal. This mismatch generally occurs as a result of amplitude variations in the information signal and differential delay variations between the information signal and the additional signal and causes instantaneous overshoot and undershoot on the modulated carrier. When the overshoot and undershoot are great, radio devices for receiving the transmitted carrier signal cannot reliably decode the signal, causing signals to be erroneously received or missed entirely. Therefore, external calibration equipment must be used to periodically calibrate the modulated carrier. In high data rate systems however, only very accurate, and therefore expensive, calibration equipment can detect the errors which result from the overshoot and undershoot. This expense is reflected both in the cost and complexity of the transmitting devices, resulting in higher priced transmitting systems and system service.
Thus, what is needed is a method and apparatus for correcting errors in the modulated carrier of a transmitting device without the use of expensive external calibration equipment.